The invention relates to a magnetic-resonance imaging (MRI) apparatus comprising a system of electric coils for transmitting and receiving radio-frequency (RF) magnetic fields in an examination volume of the MRI apparatus, said system comprising at least one surface coil extending substantially in a plane for transmitting and/or receiving a RF magnetic field directed substantially perpendicularly to the plane of the surface coil, in which the surface coil has a main coil axis which, in an operating state of the system, is oriented substantially parallel to the direction of a main magnetic field in the examination volume of the MRI apparatus, and in which the surface coil has an electrically conducting element on both sides of the main coil axis, which element extends substantially parallel to the main coil axis.
The invention also relates to a system of electric coils for transmitting and receiving RF magnetic fields in an examination volume of such a MRI apparatus.
Magnetic-resonance imaging (MRI) apparatuses of the type described in the opening paragraph and systems of electric coils for transmitting and receiving radio-frequency (RF) magnetic fields in an examination volume of such a MRI apparatus are generally known. MRI apparatuses can be used for imaging the internal parts of a patient's body, using the principle of nuclear spin resonance. A MRI apparatus comprises an examination volume, a main magnetic system for generating a uniform and relatively strong main magnetic field in the examination volume, a gradient coil system for generating gradients of the main magnetic field in three orthogonal directions, and a system of electric coils for transmitting and receiving RF magnetic fields in the examination volume. The system of electric coils for transmitting and receiving RF magnetic fields in the examination volume of a MRI apparatus may comprise a coil system which both transmits and receives RF magnetic fields, or a first coil system which comprises at least one transmitting coil for transmitting RF magnetic fields and a second coil system which comprises at least one receiving coil for receiving RF magnetic fields.
The simplest embodiments of a system of electric coils for transmitting and receiving RF magnetic fields comprise surface coils. Surface coils are generally known. They extend substantially in a curved or non-curved plane. Surface coils are often used as receiving coils because they can be positioned so close to a patient that signals having a better signal-to-noise ratio can be realized than when using other types of coils for receiving RF magnetic fields. Surface coils may also be used as transmitting coils for transmitting RF magnetic fields. If desired, some types of surface coils may be formed in a flexible way so that they can be wound around a patient to some extent.
Known surface coils for transmitting and/or receiving RF magnetic fields have the drawback that they transmit a RF magnetic field which is non-homogeneous in the volume and/or have a sensitivity to RF magnetic fields which is non-homogeneous in the volume. RF magnetic fields which are non-homogeneous in the volume and are transmitted by the surface coil, as well as the sensitivity to the RF magnetic fields received by surface coils, which sensitivity is non-homogeneous in the volume, increase as the distance to the electrically conducting elements extending substantially parallel to the main coil axis of the surface coil decreases, the largest increase taking place in the proximity of these electrically conducting elements. Consequently, a signal having a relatively satisfactory signal-to-noise ratio can be obtained from tissue proximate to the surface coil, whereas the signal-to-noise ratio of the obtained signal becomes worse as the signal comes from tissue which is further remote from the surface coil. Consequently, MR images obtained by means of such simple surface coils exhibit artefacts which are very disturbing due to the fact that one is mostly interested in tissue which is further remote from the surface coil.